Wood particles, flakes, and chips have long been optimized as feedstocks for various industrial uses (see, e.g., U.S. Pat. Nos. 2,776,686, 4,610,928, 6,267,164, and 6,543,497), as have machines for producing such feedstocks.
Optimum feedstock physical properties vary depending on the product being produced and/or the manufacturing process being fed. In the case of cellulosic ethanol production, the feedstock should be comminuted to a cross section dimension of less than 6 mm for steam or hot water pretreatment, and to less than 3 mm for enzymatic pretreatment. Uniformity of particle size is known to increase the product yield and reduce the time of pretreatment. Uniformity of particle size also affects the performance of subsequent fermentation steps.
Piece length is also important for conveying, auguring, and blending. Over-length pieces may tangle or jam the machinery, or bridge together and interrupt gravity flow. Fine dust-like particles tend to fully dissolve in pretreatment processes, and the dissolved material is lost during the washing step at the end of preprocessing.
Particle shape can be optimized to enhance surface area, minimize diffusion distance, and promote the rate of chemical or enzyme catalyst penetration through the biomass material. Such general goals have been difficult to achieve using traditional comminution machinery like shredders, hammer mills, and grinders.
Gasification processes that convert biomass to syngas present a different set of constraints and tradeoffs with respect to optimization of particle shape, size, and uniformity. For such thermochemical conversions, spherical shapes are generally favored for homogeneous materials, and enhancement of surface area is less important. Cellulosic plant derived feedstocks are not homogeneous, and thus optimal properties involve complex tradeoffs.
A common concern in producing all bioenergy feedstocks is to minimize fossil fuel consumption during comminution of plant biomass to produce the feedstock.